Abstract: Quantum transitions between different vacua may be important in the early universe; for instance our universe could directly arise from such a transition followed by a period of inflation. In this talk I will consider in such a context transitions which change the number of large dimensions of the universe, and discuss their observational signatures on the cosmic microwave background. I will also briefly discuss a class of O(3) symmetric vacuum decays which have been argued to be competitive with bubble collisions in the early universe.